This invention is generally directed to conductive particles and processes for the preparation thereof. More specifically, the present invention relates to conductive particles containing a polymer matrix and a carbon black dispersion, wherein the carbon black is evenly distributed throughout the polymer matrix using a block copolymer that contains a polymer block that is miscible with the polymer matrix and a second block that anchors to the surface of the carbon black.
In electrostatographic imaging and related development processes, images are developed using a developer generally comprising colored toner particles and carrier particles. Carrier cores generally comprise metals, which are conductive or semiconductive materials. Polymeric materials are generally used to coat the surface of the metals and are usually insulating. Therefore, carrier particles coated completely with polymer or a mixture of polymers can lose their conductivity and become insulating. Although this is desired for some applications, other applications require the carriers to have specific conductivity properties. Conductive magnetic brush development systems, for example, require carrier particles that are conductive. Since the carrier polymer coating can also control the triboelectric charging properties of the carrier, a conductive carrier coating is needed to design carriers with the desired conductivity and triboelectrical properties. However, traditional conductive polymers are very costly, and are not suitable for preparing low cost coatings. Thus, a conductive polymer composite comprising a low cost polymer and a conductive filler, such as conductive carbon black, is considered a more suitable alternative.
The coating of metal materials, such as carrier beads, with a polymer is known and can be achieved by two general approaches, solution and powder coating. Solution coating of carriers using a polymer composite solution comprised of a polymer, a conductive filler and solvent can be utilized to prepare a conductive carrier. However, trapping of solvent in the solution coating adversely interferes with the use of the coated materials. For example, residual solvent trapped in the carrier coating reduces the carrier life and the release of the solvent in the developer housing can cause other problems due to the harmful effects of adsorbed solvent on various copying machine parts and the toxicity of solvent. Moreover, the solvent recovery operation involved in solution coating processes is costly.
The powder coating of metal surfaces such as the carrier cores can eliminate the need for solvents and, therefore, many of the problems associated with solution coating. However, powder coating requires polymer powder that is very small in size, for example less than one micron. Although several polymer powders with desired particle size are available for carrier powder coating, there is a need for very small conductive polymeric particles, particularly, submicron particles, containing a conductive filler distributed evenly throughout the particles.
The preparation of polymeric particles for powder coatings can be accomplished primarily by three methods, namely grinding or attrition, precipitation and in situ particle polymerization. In grinding or attrition processes, especially fluid energy milling, large polymeric particles or polymeric composite particles containing fillers are reduced to the size needed for powder coating, for example less than one micron. However, such processes are often not desirable both from an economic and functional viewpoint. These materials are difficult to grind, and therefore, grinding or attrition of the required materials for coating with present milling equipment is very costly due to very low processing yield, for example in the range of 5 to 10 weight percent.
Precipitation processes can also be used to prepare polymeric and polymeric composite particles. In one approach, the polymer solution is heated to above its melting temperature and then cooled to form particles. In another process, the polymer solution is precipitated using a nonsolvent or the polymer solution is spray dried to obtain polymeric and polymeric composite particles. With all of these precipitation processes, it has been difficult to achieve low cost and clean polymer particles, that is, for example, with no or substantially no impurities such as solvents or precipitants in the resulting polymer particles. Furthermore, it is also difficult to obtain particles with small particle size and narrow particle size distribution. It is also difficult to control filler distribution throughout each particle's polymer matrix.
Suspension polymerization can be utilized to prepare polymer particles and polymeric composite particles containing, for example, a conductive filler. The main advantage of suspension polymerization is that the product may easily be recovered. Therefore, such a process is considered economical. However, it is very difficult to prepare very small particles by suspension polymerization, for example having a size less than five microns, because the monomer droplets tend to coalesce during the polymerization process, especially in the initial stage of polymerization where the droplets are very sticky.
U.S. Pat. No. 4,835,084 discloses a method for preparing pigmented particles wherein high concentrations of silica powder are used in the aqueous phase to prevent coalescence of the particles. U.S. Pat. No. 4,833,060 discloses a process for the preparation of pigmented particles by dissolving polymer in a solvent that is immiscible with water and dispersing the solution that is formed thereby in an aqueous phase containing silica powder to prevent coalescence of the particles. However, the silica powder used in both of these processes is removed using KOH, which is costly, and residual KOH and silica materials left on the surface of the particles affect the charging properties of the particles. Moreover, the above processes do not teach the preparation of submicron conductive particles.
In in situ particle polymerization processes, polymer particles are prepared by using suspension, dispersion, emulsion and semisuspension polymerization processes. Although emulsion and dispersion polymerization processes can be utilized to prepare polymeric particles smaller than one micron by nucleation and growth, these processes do not readily enable synthesis of particles containing fillers such as conductive fillers. Conductive fillers, such as carbon blacks, are free radical polymerization inhibitors and thus tend to reduce the rate of polymerization in such processes.
U.S. Pat. No. 5,043,404, the disclosure of which is totally incorporated herein by reference, discloses a semisuspension polymerization process for the preparation of small polymeric particles that are comprised of a mixture of monomers or comonomers, a polymerization initiator, a crosslinking component and a chain transfer component, which are bulk polymerized until partial polymerization is accomplished. The resultant partially polymerized monomer or comonomers is dispersed in water containing a stabilizer component with, for example, a high shear mixer. Then, the resultant suspension is polymerized, followed by washing and drying the submicron polymeric particles. However, the patent does not disclose submicron conductive polymeric particles containing fillers.
U.S. Pat. 5,236,629 discloses a process for the preparation of submicron particles using the semisuspension polymerization process. However, when carbon black is used as a conductive filler with monomers that do not have a high affinity for the carbon surface, particularly methacrylates and acrylates, the carbon black dispersion tends to be poor because of the difficulty in (1) dispersing the carbon black uniformly into the monomer initially (prior to polymerization) and (2) maintaining a stable carbon black dispersion (i.e. preventing formation of aggregates and clusters) during polymerization. "Poor carbon black dispersion" means that (1) some of the submicron particles contain very little or no carbon black, and/or (2) the carbon black present in the particles is not uniformly distributed within the particle, but rather is present as clusters. Either condition results in lower conductivity than is achieved when the carbon black is distributed uniformly throughout the submicron polymeric particles.
U.S. Pat. No. 5,484,681 describes a process for the preparation of submicron conductive particles that uses a diblock copolymer to tailor the triboelectric charge of the particle. This is accomplished by selecting the two blocks such that they diffuse to the particle surface during polymerization and thus have a significant effect on charge. However, the application does not disclose improving carbon black dispersion.
Even or homogeneous distribution of fillers such as carbon black is not believed achievable with the prior art processes mentioned herein. In fact, in prior art processes, the conductive filler is agglomerated around some of the polymeric particles and many of the other polymeric particles contain little or no conductive filler. Therefore a need exists for a invention to enable preparation of submicron conductive particles with even carbon black dispersion.